JP2002120387A - Multipass ink jet printing using print masking - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method of multipass ink jet printing for forming a high quality image. SOLUTION: In a method of multipass ink jet printing of an image in response to a digital image having a code value, at least two ink drops can be printed on each printing position in different passes. The method comprises a step for providing an ink jet printer including a print head capable of selecting at least two dimensions of the ink drops and a step wherein a number of passes for forming an image is selected, a printing mask having a mask value sequentially corresponding to a predetermined pixel at each pass is provided and the ink jet printer is operated such that the dimension of the ink drop with respect to each pixel at each pass is selected in response to the mask value and the code value of the pixel.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to multi-pass ink jet printing.

[0002]

2. Description of the Related Art A typical ink jet printer ejects small ink droplets from a print head having nozzles, and the ink droplets land on a receiving medium (generally paper) so as to form ink dots. Reproduce the image. A typical inkjet printer reproduces a color image by using a set of color inks, usually cyan, magenta, yellow, and black. In the field of inkjet printing,
It is known that when ink drops placed adjacently on a page are printed simultaneously, the ink drops tend to flow together on the surface of the page before soaking into the page. by this,
It gives the reproduced image a grainy or garish appearance of undesirable particles, commonly referred to as "coalescence".

It is known that the amount of coalescence in a printed image is related to the amount of time that elapses between printing adjacent dots. As the time delay between printing adjacent dots increases, the amount of coalescence decreases, thus improving image quality. In the prior art, "interlacing",
There are many techniques that describe how to increase the time delay between printing adjacent dots using a method called "print masking" or "multi-pass printing." Techniques for reducing one-dimensional periodic artifacts or "bands" also exist in the prior art. This advances the paper one increment less than the width of the printhead, thereby causing successive passes or swaths of the printhead.
Are made to overlap. Generally, print masking and pass-through techniques are combined. See, for example, U.S. Patent Nos. 4,967,203 and 5,992,962. The term "print masking" generally means printing a print subset of image pixels in multiple passes of a printhead to an image receiving medium.

[0004] Another characteristic of modern ink jet printers is that they generally have the ability (over some range) to vary the amount of each ink deposited at a given location on a page. Inkjet printers having this capability are referred to as "multi-tone" inkjet printers because they are capable of forming multiple density tones at each location on a page. By changing the electrical signal sent to the nozzle, or by changing the diameter of the nozzle
Some multi-tone inkjet printers change the volume of ink droplets formed by nozzles to achieve multiple density gradations. See, for example, U.S. Patent No. 4,746,935. Multi-tone inkjet printers are also intended to form a variable number of small fixed size droplets ejected by nozzles, all droplets coalescing together and aiming at the same location on a page. is there. See, for example, U.S. Patent No. 5,416,612.

[0005] These techniques allow the printer to vary the size or optical density of a given ink dot forming a range of density levels at each location, thereby improving image quality.

[0006] Ink jet printers that are not multi-gradation
Another common way to achieve multi-density levels is to print a small amount of ink for a given location in several different passes of the printhead at that location. This builds up ink over a number of passes at a given location, thus creating a greater number of density levels than the nozzles can originally eject. For example, US Pat. No. 5,923,3
See No. 49.

In all of the above-described ink jet printers, printer designers are further challenged with separating the image data into multiple memory buffers corresponding to multiple passes of the printhead.

[0008] An improvement over the prior art in the area of multi-pass printing, which supports both multi-tone and non-multi-tone inkjet printers that eject a large number of drops to a given location over several different passes. There is a need. Furthermore, this possibility must be combined with print masking. This is necessary to minimize coalescence and maximize ink jet printer image quality by preventing excessive use of ink.

[0009]

SUMMARY OF THE INVENTION It is an object of the present invention to provide a multi-pass ink jet printing for forming a high quality image.

[0010]

SUMMARY OF THE INVENTION An object of this invention is to provide a) a printhead that is capable of selecting at least two different ink drop sizes, wherein at least two ink drops at each printing location during different passes. Providing an ink jet printer capable of printing an image; b) selecting a number of passes for forming an image, providing a print mask having a mask value sequentially corresponding to a predetermined pixel in each pass; Operating the inkjet printer to select an ink droplet size for each pixel in each pass so that the inkjet printer forms an image in response to the mask value and the code value of the pixel. This is achieved by a multi-pass inkjet printing method of the image in response to the image.

The present invention uses an ink-jet printer capable of ejecting ink in a number of printing passes at each given location, the volume of ink or ink to be printed at a given location on an image receiving medium. Determine the drop size.

It is an advantage that the present invention can be applied to both multi-tone and non-multi-tone inkjet printers.

[0013] It is another advantage of the present invention that print masking to minimize coalescence is achieved.

It is another advantage of the present invention that the effects of the bands can be reduced by overlapping the steps.

It is another advantage of the present invention that many types of ink and media combinations can be accommodated.

[0016]

DETAILED DESCRIPTION OF THE INVENTION The present invention describes a method for printing a high quality digital image on an image receiving medium using an ink jet printer using multiple printing passes. Referring to FIG. 1, an inkjet printer system is shown, wherein an image preprocessor 10 receives a digital image from a host computer (not shown) and forms a multi-toned image signal i, for example, Standard image processing such as sharpening, scale adjustment, color conversion, and multi-gradation are performed. The multi-gradation image signal i is composed of a set of color data planes, hereinafter referred to as color channels. Each color channel corresponds to a particular pigment in the printer, such as cyan, magenta, yellow, or black used in a typical inkjet printer. The data making up each color channel is each picture element, or a two-dimensional array of "pixels" (width = w, height = h). The location of the pixel in the image is specified by the (x, y) coordinates in the array, where 0 < x < w.
−1 and 0 < y < h−1. The x position of the pixel is
The y-position of a pixel is also referred to as a row coordinate value and the y-position of a pixel is also referred to as a column coordinate value. The term "signal" generally refers to an array of pixels having digital code values that form an image.

Next, a stroke extractor processor 20 receives the multi-toned image signal i and controls the volume of ink printed by the inkjet printhead (or printheads) 30 by a printhead image signal. o is generated. Print masking and multi-pass drop formation are included in this stroke extractor processor 20 and are described in more detail below. Prior to multitoning, each pixel is a numeric code value (typically $ 0,) indicating the amount corresponding to the pigment to be placed at the location of a given pixel in the image.
255 °). After multitoning (at the output of image preprocessor 10), the range of pixel code values is reduced to match the number of density levels that the inkjet printer can produce. In a binary inkjet printer, the pixel code value is either 0 or 1, indicating whether zero or one ink drop is to be printed. Multi-tone inkjet printers accept pixel code values in the range of {0, N-1}, where N is the number of density levels that the multi-tone inkjet printer can produce.

Referring to FIG. 2, details of the stroke extractor processor 20 are shown. A "one pass" of data is defined as a subset of the multi-toned image signal i required during a single operation of the printhead across a page. FIG.
During, the mask table processor 40 receives the mask table 50 and the (x, y) coordinates of the current pixel being processed to form a mask value m. In a preferred embodiment of the present invention, mask table 50 may be stored in a data file residing on a disk storage medium in a computer running stroke extractor processor 20. In another embodiment of the present invention, having a stroke extractor processor 20 running in a computer embedded in an inkjet printer, the mask table 50 may be stored in programmable memory in the printer. Those skilled in the art will recognize that many different hardware configurations for the stroke extractor processor 20 and many different storage options for the mask table 50 may be configured, and the present invention may be adapted to any of the different configurations.

In a preferred embodiment of the present invention, the mask value m
Is calculated according to the following equation:
Calculated by 0.

M = mask table (x% m_x, Y% m_y) (EQ1) where m_xIs the number of rows in the mask table 50, and
One, m_yIs the number of columns in the mask table 50, and x is the current
The row coordinates of the current pixel, and y is the current pixel
The column symbol is the mathematical modulo operator.
is there. FIG. _x= M_y= 2 mask table
50 examples are shown. Therefore, the mask value m is equal to the current pixel value.
Using the (x, y) coordinates of the
Therefore, it can be obtained simply from the mask table 50.

Referring again to FIG. 2, the pass table processor 60 includes a mask value m, a multi-graded image signal i, a pass table 70, and a first pass index p to form a printhead image signal o. , And the number of paths Np. The first pass index p is a simple integer counter that counts the number of movements of the printhead across the page. Typically, the first pass index p is set to zero at the beginning of printing and increases by one each time a stroke of data is formed by the stroke extractor processor 20. The first path index p may be generated and stored within the stroke extractor processor 20 as it is calculated and used by the stroke extractor processor 20. The number of paths Np is set to be equal to the number of columns in the path table 70. The number of paths Np can be stored internally in a similar manner to the first path index p, as it is calculated by the stroke extractor processor 20 and used only by the processor 20.

The path table 70 is a two-dimensional lookup table including the value of the printhead image signal o,
This table can be stored in and retrieved from various physical locations as described above for the mask table 50. FIG. 3 shows a sample path table 70. The data values contained in the path table 70 can be of a variety of different forms depending on the signals that are designed to be accepted by the circuitry that operates the printhead. For example, the electrical circuitry that operates the printhead can be designed to receive a drop volume in picoliters. Thus, the circuitry that operates the printhead converts the printhead image signal o, which contains the desired drop volume, into an electrical signal that instructs the printhead to form the desired volume. It should be noted that the format of the data values in the path table 70 is not essential to the present invention, and the present invention uses the appropriate data values in the path table 70 to enable the printer image signal o for any particular printhead. It is important that it can be applied to form

Still referring to FIG. 2, the value of the printer image signal o for the current pixel is retrieved from the two-dimensional path table 70 by calculating the row and column indices used to address the table. You. The row index is calculated from the multi-graded image signal i value according to row = i (x, y) (EQ2), where (x, y) is the coordinates of the current pixel. The column index is calculated according to the following equation:

Column = (p + m)% N _p (EQ3) where p is a first pass index, m is a mask value, and Np is the number of passes. The printer image signal o for the current pixel is obtained from the path table 70 by indexing the table with row and column indices.

O (x, y) = path table (row, column) (EQ4) Referring to FIG. 5, a print example illustrating the method of the present invention is shown. FIG. 5 shows a 4 × 6 pixel section 80 of the multi-graded image signal i. In this example, a multi-tone ink jet printer capable of forming five density levels indicated by values of 0 to 4 is used for the multi-tone signal i. The multi-tone signal i is shown in FIG.
Recall that it is formed by the image preprocessor 10 therein. Referring again to FIG. 5, four passes 90, 92, 94, and 96 of the printhead image signal o are shown, using the pass table of FIG. 3 and the mask table of FIG. Is formed according to Next, these four strokes 90, 92, 94, and 96
Is printed over the same area of the page by four consecutive passes of the printhead to form the desired output image. For further explanation, how a single pixel of a multi-toned image signal i is divided into four passes 90, 92, 9
4 and 96. As an example, consider the lower left pixel of the 4 × 6 section 80 of the multi-toned signal i having the value “2” at position (x, y) = (3,0). Using this position and the mask table of FIG. 4, a mask value is calculated according to EQ1.

[0026] m = the mask table _{(x% m x, y%} m y) = mask table (3% 2,0% 2) = Number Np = 4 of the mask table (1,0) = 1 pass, Figure 3 Is the number of columns in the path table (excluding index columns). The row table row index is EQ
Calculated according to 2.

Row = i (x, y) = i (3,0) = 2 In the first pass, since the first path index p has a value of 0, the column index is calculated according to EQ3.

Column = (p + m)% Np = (0 + 1)% 4 = 1 The value of the printhead image signal o for pass 0 is EQ
4 from the path table.

O = path table (row, column) = path table (2,1) = 8 In the following three passes, the first path index is 1,
It has values of 2 and 3, respectively. This, when calculated according to EQ3, results in column indices 2, 3, and 0, respectively. The value of the corresponding printhead image signal o is
It is calculated according to EQ4 and becomes 0, 0, and 32, respectively. Therefore, the multi-graded signal i having a value of 2
In four passes 90, 92, 4 in FIG.
8, as shown in the lower left corner of 94 and 96,
It is converted to a printhead image signal o having a value of 0, 0, 32.

It should be noted that the four passes of the printhead image signal o are conceptually printed (for demonstration purposes) by four consecutive operations on the same area of the page of the printhead. There are other ways of interlacing that can be applied equally well. For example, instead of four printhead passes over the same area of the page before the page is moved, the page may be advanced one quarter of the printhead height after each pass. is there. This is explained in the prior art references on print masking and interlacing referred to above, but this was not shown in the description of the present application in order to more clearly describe the invention. However, the invention applies equally well to all types of printing, including print masking and interlaced stroke printing.

It will be apparent to those skilled in the art that the present invention can be applied to each color channel of a color ink jet printer. In this case, each color channel has its own pass table and mask table.

Referring to FIG. 6, there is shown another embodiment of the present invention in which additional processing is included to select a path table to be used in path table processor 60.

The present embodiment is useful for a printer capable of printing with a large number of raster grids to reproduce an image. Printing with multiple raster grids is common in ink jet printing where the spacing between nozzles on the printhead is greater than the desired spacing between pixels on a page. For example, a printhead having a nozzle spacing of 300 nozzles per inch (NPI) has four 300 DPI rasters, one of which is referred to as a "reference raster" and the other as a "shifted raster." By interleaving, the number of dots per inch (DPI) is 60
0 can be used to print an image. It should be noted that high resolutions, eg, 900 DPI or 1200 DPI, can be printed by incorporating more shifted rasters. The printer advantageously uses a different pass table for each available raster to provide the best image quality. The manner in which different path tables are selected and used for printing is described below.

Referring again to FIG. 6, the shifted raster processor 120 performs the shifted raster table 1 operation.
10, and as described in the above embodiment, used to form the selection value r in response to a first pass index p, which is an integer counter that counts the number of printhead movements across the page. You. Shifted raster processor 120
Calculates the row index in response to the first pass index p and the number of passes Np, and then indexes the raster table 110 shifted by the row index to form a selection value r according to the following equation: The selected value is extracted from the shifted raster table 110.

Row = (p)% Np (EQ5) r = shifted raster table (row) (EQ6) FIG. 7 shows an example of the shifted raster table 110. In FIG. 7, the selection value indicates whether to use the reference raster (0) or the shifted raster (1) for the current print pass. With further reference to FIG. 6, the path table selector 130 receives the selection value r, and by simply selecting the path table corresponding to this selection value,
A path table to be used for this path is selected from the path table set 100. Next, the selected path table is received by the path table processor 60,
The printhead image signal o is calculated in a manner similar to that described in the first embodiment. The only other difference between the first embodiment and the second embodiment is that the path table processor 60 receives the second path index pl (y) instead of the first path index p. is there. The second pass index pl (y) is an integer counter that is maintained separately for each row of the image, and simply counts whether the printhead nozzle has passed directly over a given number y of rows, or simply counts that number. I do. The second pass index is necessary because the first pass index counts the overall number of printhead movements, regardless of which raster the printhead nozzles are aligned with.

[Brief description of the drawings]

FIG. 1 is a block diagram illustrating an inkjet printer system into which the present invention is incorporated.

FIG. 2 is a block diagram showing details of a stroke extractor of FIG. 1;

FIG. 3 is a diagram illustrating an example of a path table in FIG. 2;

FIG. 4 is a diagram illustrating an example of a mask table in FIG. 2;

FIG. 5 illustrates the application of the method of the present invention to a 4 × 6 section of multi-toned image signal i-pixels.

FIG. 6 is a diagram showing another embodiment of the present invention.

FIG. 7 is a diagram showing a raster shift table.

[Explanation of symbols]

 Reference Signs List 10 image preprocessor 20 stroke extractor processor 30 inkjet printhead 40 mask table processor 50 mask table 60 pass table processor 70, 100 pass table 110 shifted raster table 120 shifted raster table processor 130 pass table selector

 ──────────────────────────────────────────────────の Continued on front page (72) Inventor Rodney El Miller New York 14450 Fairport Cypress Circle 5F Term (Reference) 2C056 EC11 EC12 EC41 EC71 EC74 ED01 FA10 2C057 AF39 AM03 AM15 AM30 CA01

Claims (3)

[Claims] 1. A method of multi-pass inkjet printing of an image in response to a digital image having code values, comprising: a) a printhead capable of selecting at least two different ink drop sizes. Providing an ink jet printer capable of printing at least two drops of ink at each printing location during a pass; b) selecting a number of passes to form an image, with a predetermined pixel in each pass; Providing a print mask having sequentially corresponding mask values and, in response to the mask values and the code values of the pixels, selecting the ink drop size for each pixel in each pass so that the inkjet printer forms an image. Operating the inkjet printer to perform the operation. 2. Determining the volume of ink to be printed on a receiving medium at each pixel location and reproducing a digital image including an array of pixel code values by using an ink jet printer capable of multiple printing passes. A) calculating a row index for a first look-up table in response to each pixel code value; b) a position of each pixel, a first pass index, and a number of passes. C) calculating a column index for said first look-up table in response to said first look-up table, whereby said first look-up table provides information about the ink drop size for each pixel in each pass; Index the first look-up table with the row index and the column index for each pixel in Determining the ink volume to be printed at a given pixel location; and d) printing the ink volume using the inkjet printer at each pixel location in each pass to reproduce the image. A method that includes 3. Reproducing a digital image containing an array of pixel code values by determining an ink volume to be printed on a receiving medium at each pixel location and using an ink jet printer capable of multiple printing passes. A) selecting a first look-up table from a set of look-up tables in response to a first path indicator; and b) selecting the first look-up table in response to each pixel code value. 1
Computing a row index for the look-up table of c), the given pixel location, a second pass index, and
Calculating a column index for the selected first look-up table in response to a number of passes, whereby the selected first look-up table provides information regarding the drop size for each pixel in each pass. D) indexing the selected first look-up table by the row index and the column index for each pixel in each pass, thereby ink to be printed at the given location Determining a volume; and e) printing the ink volume using the inkjet printer at each pixel location in each pass to reproduce the image.